JP2535040B2 - A method for reducing hydrocarbon emissions in the depressurization process of a high-pressure polymerization reactor. - Google Patents

Abstract

The pressure in a continuous high pressure polymerization system, in which ethylene, with or without comonomers, is polymerized under high pressures and temperatures and the unconverted amounts of gas are recycled through the high pressure recycle gas system into the polymerization system, is rapidly reduced to below the particular reaction pressure when predetermined limiting pressure and/or temperature values are exceeded or when other faults occur by opening one or more pressure-relief apparatuses mounted on the high pressure polymerization system and transferring the hot let-down reaction mixture from the polymerization system via one or more let-down lines through one or more separator systems into the atmosphere, by a process in which the high pressure polymerization system is divided into a plurality of isolated sections at the same time as the let-down process is triggered and only the section in which the let-down process is triggered is let down to the atmosphere. Preferably, the sections in which the let-down process is not triggered can be let down into the high pressure recycle gas system, if necessary through a product separator, in synchronization with triggering of the let-down to the atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides that the polymerization of ethylene or a mixture of ethylene and a compound copolymerizable therewith is carried out at a pressure of 500 to 5000 bar and a temperature of 150 to 400 ° C. and is unreacted. The present invention relates to a method for rapidly reducing the pressure in a continuously operated high-pressure polymerization reaction system in which gas is released to 200 to 400 bar and then returned to the polymerization reaction system by a high-pressure circulating gas system.

In this method, the pressure opens one or several pressure relief devices provided in the high-pressure polymerization reaction system when the pressure and / or temperature value exceeds a predetermined value or in the case of other troubles, and The pressure below the reaction pressure is reduced by transferring the hot reaction mixture from the polymerization reaction system to the atmosphere by means of one or several pressure relief conduits and through one or several separation systems.

In this type of process, means are needed to avoid the high pressures and / or temperatures in the high-pressure polymerization reaction system that occur during decomposition, which can be an obstacle, for example in the case of tubular reactors. For this purpose, the supply of the initiator is immediately stopped and the reaction system is depressurized to a pressure much lower than the reaction pressure. This slows down the strongly pressure-dependent polymerization reaction. For safety reasons, it is not permissible to collect hot reaction mixtures from ethylene, polyethylene and decomposition products liberated during decomposition or other emergency shutdowns in closed vessels. The depressurization of the reactor is carried out in the atmosphere, releasing a hot mixture of polyethylene, ethylene, soot and decomposition products. In addition, environmental loading caused by solid materials forms gas clouds from hydrocarbons such as ethylene, methane, hydrogen and soot, which can mix with air and cause explosive ignition. This explosion-like ignition of the gas cloud is extremely dangerous to the equipment and its vicinity and should be avoided.

Ventricular tubing and liquid in the outflow tract of the gas (which is injected at the time of pressure relief to cool the gas) in order to satisfy the safety required during pressure relief of the high-pressure polymerization reaction system. It is known to attach to this system a device consisting essentially of the containing storage container (see GB 1393919). The liquid may be water, for example.

According to GB-A-827682 there is known an ethylene high pressure polymerization process which uses a device with an additional valve directly connected to the polymerization chamber, which in the case of overheating allows the continuous polymerization of normal polymerization products. The decomposition products formed can be removed directly by means of pressure relief vessels without disturbing or having to interrupt the mechanical finishing process. In this method, solid decomposition products are collected separately, but no separate device is provided for cooling and purification of the resulting gaseous decomposition products.

Other methods of producing ethylene polymers under high pressure and high temperature allow the reaction mixture to be effluent when a certain pressure is exceeded, by means other than by means of an ordinary outflow conduit, with a safety device operated at this pressure. The reactor mixture, which is discharged via a tube and flows out, is mixed with water (see US Pat. No. 3,781,256). For this purpose, several chambers, which are completely or partially filled with water, which are opened at the prevailing temperature and pressure conditions, are provided in the chamber associated with the pressure relief conduit. . The container containing this water is a small chamber made of thermoplastic synthetic resin. That is, in this method, since the cracked gas taken out from the top is mixed with water, the hot reaction mixture and the decomposed products can largely avoid explosion-like ignition into the air.

Further, according to U.S. Pat. No. 4,115,638, the radiation of the hot depressurized reaction mixture exiting the pressure relief conduit is partially submerged in water at an angle of 0 to 45 ° with respect to the water surface rather than below it. It is introduced into a filled pressure relief vessel, where the radiant flow is brought into contact with or plunges into the water surface.

According to the literature, a number of methods are also known in which the entire contents of the polymerization reaction system are released into the atmosphere, whereby the danger of explosion in releasing the reaction mixture to the outside is prevented (US Pat.
39412, 4424319 and 4534924 each specification).

A disadvantage of the known process for rapidly reducing the pressure in a continuously operated high-pressure polymerization reaction system is the release of large amounts of hydrocarbons such as ethylene. In the case of the actually used automatic switchgear, which is necessary for the rapid removal of the reaction mixture during decomposition, the entire reactor is pressure-released, i.e., the entire reaction mixture under high pressure. Operate to atmospheric pressure from the compressor to the end of the reactor or the high pressure product separator. Depending on the size of the reactor used, up to 4 tons of hydrocarbons, especially ethylene, are released in the depressurization process, which leads to a very strong environmental load.

The object of the present invention was to develop a process which makes it possible to reduce the hydrocarbon emissions which occur during the depressurization process of tubular reactors or autoclaves used for the polymerization of ethylene.

The invention relates to ethylene or a mixture of ethylene and compounds copolymerizable therewith at a pressure of 500 to 5000 bar and
After polymerizing at a temperature of 50 to 400 ° C and releasing the pressure to 200 to 400 bar, the unreacted gas is returned to the polymerization reaction system by a high pressure circulating gas system, and the planned pressure and / or temperature limit is exceeded. In the case of damage or other obstacles, one or several pressure relief devices equipped in the high-pressure polymerization reaction system are opened, and the pressure-relieved hot reaction mixture is removed from the polymerization reaction system by one or several The pressure is reduced to a pressure lower than each reaction pressure by transferring it to the atmosphere through one or several separation systems by means of the pressure release conduit of the above, and at the same time as the start of the pressure release process in several constituent sections. The pressure of the continuously operated high-pressure polymerization reaction system is rapidly increased, which is characterized by closing the high-pressure polymerization reaction system and releasing only the constituent section for which the release process is performed to atmospheric pressure. It is a method of lowering.

Rapidly reducing the pressure means that the pressure releasing device for the pressure in the high-pressure polymerization reaction system (industrialally referred to as a reactor) is operated within about 30 seconds, preferably 5 to 20 seconds, at least the initial pressure. It means to reduce to 50%. Rapid depressurization to low pressure is important and avoids reactor failure.

A mixture of ethylene or a mixture of ethylene and a compound copolymerizable therewith with a pressure of 500 to 5000 bar and a temperature of 150 to 400 ° C.
After releasing the pressure to 200-400 bar, the amount of unreacted gas is returned to the polymerization reaction system by the high pressure circulation system, and the produced ethylene polymer is separated in the high pressure product separator. Continuously operated high-pressure polymerization reaction systems are known from the literature. The method in which a tubular reactor or an autoclave reactor is used is described, for example, in Ullmann, Enticlopede del der Tehinitzien Chemie 4th edition, Vol. 19, pp. 169-226.

The pressure in the polymerization reaction system to a pressure lower than each reaction pressure,
However, the reduction to at least 50% of the initial pressure is carried out if the planned pressure and / or temperature limits are exceeded or if other obstacles occur. Exceeding the predetermined pressure and / or temperature limits may occur, for example, when the polymerization reaction is converted into a cracking reaction due to incorrect initiator injection or poor reactor cooling, in which case one of the reactor contents Parts are converted to soot and gaseous decomposition products such as methane and hydrogen. Due to the high reaction rate of ethylene, the reaction conditions in the reactor are close to the point where the decomposition of ethylene takes place, so that less heat is removed due to polyethylene depositing on the reactor wall, or in the reactor. Due to insufficient mixing of the hot mixture present, the reaction conditions may reach this limit in some places, which leads to decomposition. Due to the strongly exothermic reaction which takes place in this case, the temperature and pressure rise rapidly in the reactor, in which case the pressure and / or temperature limits which are intended are exceeded. Other obstacles can occur if equipment is damaged by poor flow, or if, for example, the concentrator is down, which results in insufficient ethylene supply.

The opening of one or several pressure relief devices provided in the reactor is carried out by means of conventional pressure relief valves. Instead of a relief valve whose opening is performed above the pressure and / or temperature limits,
As a rule, it responds automatically above a certain pressure limit,
And a bursting safety device or safety valve that causes a partial discharge of the reactor can also be used. It is also possible to use pressure relief devices that act automatically or be adjusted. It is particularly advantageous to operate with a regulated pressure relief device.

Removing the hot depressurized reaction mixture from the reactor is done by one or several pressure relief conduits, all of which open into the pressure relief vessel. The pressure relief conduit is connected to the reactor as well as to the separation system, so that the gas mixture optionally leaving the reactor reaches the separation system or pressure relief vessel. The pressure relief vessel is mounted as a separator top so as to hold a pressure of at least 10 bar. Due to the open top of the pressure relief vessel, the escaping gas or gas / product mixture is either introduced directly into the atmosphere or through a similarly configured second separating top before reaching the atmosphere. In a known manner, water is injected into the escaping gas stream (see British Patent No. 1393919) and water is produced by rupturing a synthetic resin container containing water (US Pat. No. 378125).
6)) or a gas stream can be directed to the surface of the water (see US Pat. No. 4,115,638).

According to the method of the present invention, the high-pressure polymerization reaction system is closed at the same time as the start of the pressure release process in several constituent sections, and only the constituent section that has been opened for the pressure release process is released to the atmosphere. It

According to a preferred method, the component section which is not opened to the pressure release process is released into the high-pressure circulating gas system at the same time as the start of the release to the atmosphere. The method of releasing pressure through the product separator into the high pressure circulating gas system is particularly advantageous.

The pressure relief of the reaction mixture from the reactor (tubular reactor or autoclave) is released not only in the case of decomposition, but also in other cases. That is, for example, the reactor is automatically stopped even if the control device or the monitoring device is stopped, but according to the present invention, only the component section in which the abnormality occurs also releases the pressure to the atmosphere. To be done.

In the process according to the invention, numerous temperature measuring positions and several pressure measuring devices are distributed over the entire length of the reactor. When the upper limit mark is reached for pressure and temperature monitoring, the so-called emergency program is opened. This is the case, for example, in the case of disassembly, but in the case of erroneous measurements as well as in other technical defects, the last two cases being so-called false release.

The automatic switchgear that is actually used to implement the emergency program is a commercially available measuring and regulating system (eg AEG-Theoretical Calculation Regulator), which regulates a series of hydraulically actuated valves.

In the simplest case of the process according to the invention, the reactor has only one reaction zone, but multizone reactors are also used in practice. It is rare to operate with combinations from autoclaves and tubular reactors connected in series. The process according to the invention can also be used for these types of reactors (multizone reactors, autoclaves in combination with tubular reactors).

The process for the one-zone reaction described below is illustrated by the steps in FIG.

The reactor may be divided into 5 component sections,
It is operated by a hydraulically actuated shut-off valve when opening the so-called emergency program. Each structural section means the area of the reactor shown in the following table. Area of composition between valves I Compressor (NV) A and B ₁ II Start zone (STZ) B ₁ and C ₁ III Reaction zone (RZ) C ₁ and D ₁ IV Aftercooler (NK) D ₁ and E ₁ V High pressure separator (HD) E ₁ and F reactors are equipped with pressure relief valves (1 to 5) in each constituent section, and the pressure relief valve is a separation system by a pressure relief pipe (its discharge pipe is introduced into the atmosphere It is associated with. In the depressurization process, the depressurized reaction mixture from the reactor section where the cause of the opening is present passes through the separation system before reaching the atmosphere.

Components I, II, III and IV can be discharged by a discharge conduit into a high-pressure circulating gas system [short high-pressure circulation or HD-circulation (300 bar)], if there is no cause for their opening. For this purpose the hydraulic valves B ₂ , C ₂ , D ₂ or E ₂ are opened and the reaction mixture present in the corresponding component of the reactor is fed to the discharge conduit. Emission conduit is at least 1 m
It opens into a product separator (WA) with a volume of ³ . When using a container with a smaller volume,
Part of the polyethylene melt from the degassed gas / product mixture during the depressurization process, HD from the product separator (WA)
There is a risk of being transported into the adjacent cooler of the circulation, where the cooled melt will accumulate, resulting in blockage of the high pressure circulation (HD-circulation). Vessels with a volume of more than 1 m ³ can only act particularly advantageously for the collection of the reaction mixture discharged from the constituent parts of the reactor.

Component V has no coupling to the exhaust conduit as part of the high pressure circuit. If this component does not causally contribute to the opening of the pressure relief process, the shut-off valve E ₁ is closed but the valve F remains open. On the other hand, when there is a cause of opening in the structural section V, the pressure release valve 5 is opened to release the pressure to the atmosphere, and the shutoff valves E ₁ and F for this section are closed at that time.

For example, it is possible that the decomposition actually begins at the boundaries of the constituent sections and spreads to adjacent areas. In this case, both sections are released from the separation system into the atmosphere by the automatic switchgear and the remaining constituent sections are discharged into the HD-circulation.

Table 1 below shows the pressure release process of each component section of the one-zone reactor, and each position and opening / closing process of the hydraulic valve.

The description for the single zone reactor applies as well for the autoclave. The section shown as the start zone (STZ) in FIG. 1 is used as it is at the start of the autoclave, but its function changes during the operation of the cooling zone. The autoclave itself can be considered as Category II in FIG.

For multi-zone reactors, the principles described for single-zone reactors apply analogously, with the addition of some subsections. For each of the other reaction zones, the number of components is increased by 2 (reaction zone and its associated start zone) or by 3 if an additional compressor is used.

Also for the multi-zone reactor, each component is equipped with a shut-off valve for partitioning, a pressure relief valve to the atmosphere and a pressure relief valve to the high pressure circulation (component V above) (see Figure 2).

The depressurization process of the individual components for the dual zone reactor and the respective position and opening and closing processes of the hydraulic valve are shown in the process diagram of FIG.

In the examples below, the results of the process according to the invention are shown for a one-zone reactor charged with 30 t / h of reaction mixture and a two-zone reactor charged with 20 t / h of reaction mixture, without the device according to the invention. This will be described in comparison with the pressure release process in the same reactor.

In the single-zone reactor shown in FIG. 1, according to the invention, 7.8 to 56.25% by weight of the mixture present in the whole reactor
Is only released when the pressure is released into the atmosphere.

For the dual zone reactor shown in FIG. 2, the reaction mixture released during the release process into the atmosphere is according to the invention
It is reduced to 4.5-50% by weight.

Examples 1-5 A single zone reactor is charged with 30 t / h of reaction mixture and 230
Operated at a pressure of 0 bar.

During the depressurization process of the individual constituents, the quantities of gases or products indicated in the table below were depressurized into the high-pressure circulation through the discharge conduit and into the atmosphere through the separator.

The amount of reaction mixture present in the entire reactor was 3.2 t. % Relates to the total weight. The process according to the invention made it possible to reduce the hydrocarbon emissions to 7.8-56.2% by weight.

Comparative Example 1 (relative to Examples 1-5) A single zone reactor with a reaction mixture throughput of 30 t / h
Operating as in Examples 1-5 at a pressure of 2300 bar. Since there was no possibility of shut-off in this reactor, the mixture of the entire reactor had to be vented to the atmosphere during depressurization.

Examples 6-12 A dual zone reactor was charged with 20 t / h of reaction mixture and 210
Operated at a pressure of 0 bar. For each component category,
Releasing pressure is started in each case, and the amount of gas shown in the following table /
The product was discharged into the high pressure circuit as well as released through the separation system into the atmosphere.

The amount of reaction mixture present in the entire reactor was 2.2 t. The process according to the invention made it possible to reduce hydrocarbon emissions to 50 to 4.5% by weight. % Relates to the total weight.

Comparative Example 2 (relative to Examples 6-12) In the same way as in Examples 6-12, the same two-zone reaction zone was operated with a reaction mixture throughput of 20 t / h and 2100 bar. The reactor had no potential interruptions during the compartments. The entire reactor mixture must be vented to the atmosphere during the depressurization process.

[Brief description of drawings]

1 and 2 are process diagrams for explaining the method of the present invention using a high-pressure polymerization reaction system having one and two reaction zones, respectively. Symbol NV in the figure is a compressor, STZ is a start zone, RZ is a reaction zone, NK is a post-cooler, HD is a high pressure separator,
WA means separator.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Klaus Pfreger, Federal Republic of Germany 5047 Vweseling Otto-Schutlerse 6 (72) Inventor Siegfried Kurzervewe, Federal Republic of Germany 5047 Vweseling Huazanen Wake 4 (72) Invention Klaus Bechtier Germany 5047 Vweseling Antonius Schutrase 5 (72) Inventor Gerhard Arnold Germany 5047 Vweseling Weiss Dornwake 4

Claims (3)

(57) [Claims] 1. A mixture of ethylene or a mixture of ethylene and a compound copolymerizable therewith at a pressure of 500 to 5000 bar and 1
Polymerization was carried out at a temperature of 50 to 400 ° C, the pressure was released to 200 to 400 bar, and then the unreacted gas was returned to the polymerization reaction system by a high pressure circulating gas system, and the planned pressure and / or temperature limit was exceeded. In the case of, or in the case of other obstacles, one or several pressure relief devices provided in the high-pressure polymerization reaction system are opened, and the hot reaction mixture of which pressure is released is released from the polymerization reaction system by one or several. The pressure is lowered to a pressure lower than each reaction pressure by transferring it into the atmosphere through one or several separation systems by means of a pressure conduit, and at the same time, the high pressure polymerization is started at the start of the pressure release process in several constituent sections. A method for rapidly lowering the pressure of a continuously operated high-pressure polymerization reaction system, characterized in that the reaction system is closed and only the constituent sections opened for the pressure release process are released to the atmosphere. . 2. The method according to claim 1, wherein the component section which is not opened for the pressure release process is released into the high pressure circulating gas system at the same time when the release of the pressure to atmospheric pressure is started. . 3. A process according to claim 2 wherein the pressure is released through the product separator into the high pressure circulating gas system.